Technically, at least where I live in Canada, you have to pay taxes on crypto income and capital gains. It's possible for goverments to track crypto transactions.
That being said, if you think normal fiat currencies are without crimes and illegal activities, you're kinda stupid.
I'm not talking about the current way, I'm talking about the possibility of a privacy-focused crypto, issued by the state, where transactions can be made private with the Blockchain. This crypto, as it would be used for normal transactions, wouldn't have more variability or speculation than the variability and speculation in converting US Dollars to UK Pounds. The post talks in hypotheticals, I do too.
I don't think fiat currency isn't used for illegal activity, I think crypto is mostly not used for normal transactions.